Good idea.
Last year I refurbished an '88 Laredo for an ex-girlfriend. The back was sagged maybe 1/2", and I knew she wanted to use it to access remote trail heads in Montana for hiking, plus be able to carry her dog in the back. So I figured a bit of reinforcement was in order.
I was given the main leaves from an XJ that another NAC member stripped. Cut them off just short of the eyes and added them into the stock packs just beneath the original main leaves, as full-length add-a-leaves. Came out sitting about 3/4" to 1" higher than "stock," which figurews out to a gross gain of about 1-1/4" from where it was -- with some extra load-carrying capacity as well. Ride was very comfortable, not at all harsh. (I drove it from Connecticut to Montana.)
One of the NAC members who comes to NAC Fest did the same thing with a 2000 XJ and I believe he gained about 1-1/2" gross.
You will need a pair of new center pins for the spring packs. The XJ uses 5/16" center pins. The stock u-bolts are long enough to handle one extra leaf. If you want more than that, you need new u-bolts. Actually, you should use new u-bolts anyway, but if you won't be abusing it too harshly one rer-use probably won't be the end of the world. Even engine head bolts get one re-use. After that, get new ones.